Heating of foods for eating induces many complex chemical and physical changes in the food. These changes and desirable conditions for enjoyable consumption are often very transient. The essential properties of temperature, texture, flavor, odor and appearance all change relatively rapidly during preparation and serving of most foods.
Microwave ovens have been employed heretofore to provide rapid internal heating of food products. However, many food products depend upon surface heating effects for texture, flavor, odor and appearance, so that microwave energy alone does not provide acceptable heating of these food products.
Infra-red radiation and hot air convection with microwave have been employed for food cooking or baking as well as for reheating previously prepared foods with limited success.
Particular difficulty has been encountered in thawing, cooking, and browning foods, such as frozen fish, meat patties, chicken parts and pies.
Microwave energy can be very effectively employed to thaw frozen foods because the heating effect penetrates deeply into the frozen food. Microwave energy passes through ice much more readily than through water in foods. For example, half power depth of microwave at 3,000 megahertz in ice at -12 degrees centigrade is 685 millimeters, while water at 26 degrees centigrade would absorb the same portion of the energy in 0.80 centimeter.
Microwave is very advantageous in thawing thick sections. However, heating of exterior surfaces of frozen food products thaws the surface, and the thin outer layer of thawed food material preferentially absorbs microwave and interferes with deep thawing. Thus, it is desirable to provide proper programming of the internal heating and surface heating when thawing frozen foods and heating them for serving.
Forced convection type heaters heretofore devised have been incapable of providing controlled transfer of heat to the surface of food products to accomplish crisping, browning, or searing within the short time required to heat food with microwave.
Forced convection ovens in which air heated to 500 to 550 degrees Fahrenheit, is circulated by a fan longitudinally through a microwave oven for heating exterior surfaces of a food product require proper shaping and composition of food portions.
Apparatus of the forced convection type generally provides insufficient external heating to the product within the time required for cooking internal portions of the product with microwave. Further limitations of forced convection heating devices heretofore devised result in burning low heat sink areas such as flaky blisters on pastries or projecting points on vegetables and meats before properly finishing high heat sink areas such as solid dough between blisters and relatively flat dense areas of meat.
The apparatus disclosed in U.S. Pat. No. 3,514,576 employed a fan to circulate heated air through to cooking compartment in a closed system to conserve heat in an effort to provide sufficiently high temperature for rapid surface heating of food products. It should be appreciated that such a system prevents turning off surface heating apparatus quickly and, if high intensity heat is employed, requires immediate removal of food products upon completion of a cooking cycle.
During the cooking process foods release grease, pyrolysis products, volatile organic materials and water which deposit on walls and surrounding parts. If these products are diluted during food heating or between heating operations, the concentration of volatile or suspended materials does not accumulate to give as much deposition on the oven surfaces as in a closed system.
It is important to note that accumulation of grease and other materials in fine particles on hot surfaces results in oxidation, rancidity and oven odor. Unless an oven is cleaned thoroughly and frequently, this odor can be transferred into new food. In addition, the accumulated materials on oven surfaces may be burned by microwave heating causing arcing in the oven and damaging parts thereof as a result of overheating.
Microwave ovens heretofore devised have offered significant advances in internal heating. However, microwave energy has not heretofore been employed in combination with apparatus to provide optimum exterior characteristics to cooked food products, particularly for crisping, searing or browning moist food.
The knowledge of separable or coincident internal and surface heating which the use of microwave makes possible leads to understanding and improvements in the state of the art of surface heating of food products whose dimensions or composition do not require the use of microwave for rapid heating.
For example, thin pizza shells of pastry with tomato paste, cheese and other toppings heat rapidly from only surface heating. Many pizza ovens have stone or low conductivity material to limit heat transfer rate to the bottom of the pizza. When the crust of the pizza blisters, as is desirable to make flaky tender crust, the blister surface is thin and browns. The blister lifts the surface of the pizza crust away from the heating surface and the more dense dough between the blisters bakes much more slowly. Thus, the thin shell of the blisters overbake and the areas between blisters bake much more slowly. One result of this effect is that pizzas must be baked relatively slowly to avoid soft dough between the crisp blisters.
Another example of food product which requires controlled surface heating, but is not necessarily assisted by microwave heating, is French fried potatoes.
Since there are no sections of the long thin potato pieces which are far removed from the outer surface thereof, the product can be rapidly warmed by surface heating. Usually this is done by frying in hot oil because it is necessary to crisp and brown the outside of the piece without excessive drying of the inside. Microwave heating of previously prepared French fried potatoes leaves them warm, but soggy and tasteless. Still, air oven heating dries the whole piece before the surface becomes crisp or brown. Infra-red heating browns or crisps only points adjacent ends of the pieces and the upper surfaces.
It should be appreciated that some of the problems which are encountered in transferring heat to a surface are also encountered in cooling or freezing products.
Freezing of foods products is usually least harmful to the product and evaporative loss is reduced when cooling is accomplished rapidly.
Cryogenic freezing, wherein liquid such as nitrogen having a very low boiling point is applied to the product, is relatively expensive because the "spent" vapor is usually discharged continually at a very low temperature.
The rate of freezing in blast freezing, wherein cold air is forced over the product, is limited by the rate of heat transfer to cold air form the product when the air travels generally parallel to the surfaces.